11,650 research outputs found
Light-Front-Quantized QCD in Covariant Gauge
The light-front (LF) canonical quantization of quantum chromodynamics in
covariant gauge is discussed. The Dirac procedure is used to eliminate the
constraints in the gauge-fixed front form theory quantum action and to
construct the LF Hamiltonian formulation. The physical degrees of freedom
emerge naturally. The propagator of the dynamical part of the free
fermionic propagator in the LF quantized field theory is shown to be causal and
not to contain instantaneous terms. Since the relevant propagators in the
covariant gauge formulation are causal, rotational invariance---including the
Coulomb potential in the static limit---can be recovered, avoiding the
difficulties encountered in light-cone gauge. The Wick rotation may also be
performed allowing the conversion of momentum space integrals into Euclidean
space forms. Some explicit computations are done in quantum electrodynamics to
illustrate the equivalence of front form theory with the conventional covariant
formulation. LF quantization thus provides a consistent formulation of gauge
theory, despite the fact that the hyperplanes used to impose
boundary conditions constitute characteristic surfaces of a hyperbolic partial
differential equation.Comment: LaTex, 16 page
Lattice thermal conduction in ultra-thin nanocomposites
This is the final version of the article. Available from the publisher via the DOI in this record.We have studied the lattice thermal conductivity of Si/Ge periodic nanocomposites (superlattice, nanowire, and nanodot structures) of sample sizes in the range of 30 nm-30 μm, periodicities 1.1 nm and 2.2 nm, with reasonably dirty interfaces, and n-type doping concentration in the range of 1023-1026m-3. Our calculations employ a judicious combination of ab initio and physically sound semi-empirical methods for detailed calculations of estimates of phonon scattering rates due to anharmonicity and interface formation. Based upon our results we conclude that the formation of ultra-thin nanocomposites in any of the three structures is capable of reducing the conductivity below the alloy limit. This can be explained as a result of combination of the sample length dependence, the on-set of mini-Umklapp three-phonon processes, mass mixing at the interfaces between Si and Ge regions, and the sample doping level.We are grateful to the EPSRC (UK) for supporting this
project via the Grant Award No.r EP/H046690/1. Quantum
Espresso calculations were performed on the Intel Nehalem
(i7) cluster (ceres) at the University of Exeter. I.O.T. also
acknowledges support from the John Templeton Foundation
as part of the Durham Emergence Project during the final
stages of this work
Shell model description of Ge isotopes
A shell model study of the low energy region of the spectra in Ge isotopes
for is presented, analyzing the excitation energies,
quadrupole moments, values and occupation numbers. The theoretical
results have been compared with the available experimental data. The shell
model calculations have been performed employing three different effective
interactions and valence spaces.We have used two effective shell model
interactions, JUN45 and jj44b, for the valence space
without truncation. To include the proton subshell in valence space
we have employed the effective interaction due to Sorlin {\it et al.},
with Ca as a core and a truncation in the number of excited particles.Comment: 10 pages, 10 figures, Proc. of the XXXV Nuclear Physics Symposium,
January 3-6 2012, Cocoyoc, Morelos, Mexico. IOP Journal of Physics:
Conference Series (in press
Quantum atomic delocalization vs. structural disorder in amorphous silicon
Quantum effects on the atom delocalization in amorphous silicon have been
studied by path-integral Monte Carlo simulations from 30 to 800 K. The quantum
delocalization is appreciable vs. topological disorder, as seen from structural
observables such as the radial distribution function (RDF). At low
temperatures, the width of the first peak in the RDF increases by a factor of
1.5 due to quantum effects. The overall anharmonicity of the solid vibrations
at finite temperatures in amorphous silicon is clearly larger than in the
crystalline material. Low-energy vibrational modes are mainly located on
coordination defects in the amorphous material.Comment: 5 pages, 5 PS figures, REVTE
Human papillomavirus 16/18 and nasopharyngeal carcinoma
Sixteen cases of nasopharyngeal carcinoma (eight anaplastic and eight well differentiated squamous types) were examined for the presence of human papillomavirus types 16 and 18 genomes using the polymerase chain reaction on paraffin wax embedded biopsy specimens. Although nasopharyngeal carcinoma, particularly the anaplastic type, is strongly associated with Epstein-Barr virus, other factors may be involved in its pathogenesis. No DNA of either human papillomavirus subtype was detected. It is concluded, therefore, that these two 'high risk' types of human papillomavirus are not implicated in the pathogenesis of nasopharyngeal carcinoma. The number of cases in this series was small, however, and further studies are warranted using fresh biopsy material and including other viral subtypes.published_or_final_versio
Optical Phonon Lineshapes and Transport in Metallic Carbon Nanotubes under High Bias Voltage
We calculate the current-voltage characteristic of metallic nanotubes at high
bias voltage showing that a bottleneck exists for short nanotubes in contrast
to large ones. We attribute this to a redistribution of lower-lying acoustic
phonons caused by phonon-phonon scattering with hot optical phonons. The
current-voltage characteristic and the electron and phonon distribution
functions are derived analytically, and serve to obtain in a self-contained way
the frequency shift and line broadening of the zone center optical phonons due
to the electron-phonon coupling at high bias. We obtain a positive frequency
shift from the zero bias shift and no broadening of the optical phonon mode at
very high voltages, in agreement with recent experiments.Comment: 16 pages, 8 figures, minor changes, pusblished in PR
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